1. Field of the Invention
The invention relates to a tube bundle heat exchanger of graphite in which a multiplicity of graphite tubes is set in tube sheets and is connected to the sheets by screw or cement connections.
2. Description of the Prior Art
Graphite, under which term carbon and carbon-graphite mixtures will also be included in the following, is relatively resistant to high temperatures and aggressive substances and exhibits practically metallic conductivity properties and accordingly is often used, also as a structural part, under conditions which metallic and ceramic materials can generally not stand. Due to their manufacture, graphite parts are porous and, if a reduction of their permeability is necessary, are impregnated with a synthetic resin or other means determined by the conditions of the application. Impregnated graphites are practically impenetrable for fluids and it is known to use these graphites, for instance, in the form of tubes in tube bundle heat exchangers.
A frequently less satisfactory property of the graphite material is its relatively low strength, particularly in the presence of shock-like and undulating stresses. Fiber-like graphites have been known for some time which do not have this disadvantage and belong among the materials with the greatest strength. Graphite fibers are preferably used for reinforcing hardenable synthetic resins and are coated for this purpose with a synthetic resin solution and are laid out or wound in this form to form planar structures. The resins are hardened by a special heat treatment and optionally, pyrolized by heating the blanks to a higher temperature. In the latter case, a carbon body reinforced by carbon fibers is obtained which has great strength and a very advantageous thermal resistance. The fiber content of these bodies is generally more than 50%. It is likewise known to process carbon fibers in the form of fabrics, tapes or the like and to orient them in accordance with the expected mechanical stresses of the finished bodies. Carbon fiber-reinforced carbon bodies are very expensive because of the high cost of the graphite fibers and the relatively elaborate production process, so that the use of this material has been limited to areas in which, as in aeronautics, weight savings are an important advantage.
It is known, German Pat. No. 19 33 369, to coat structural parts such as sheet metal pieces or tubes with a resin composition containing graphite fibers cut to a short length to improve the corrosion resistance. The coated parts have adequate corrosion resistance under certain conditions, but this solution to the problems is not always satisfactory because of the limited thermal stability. In addition, the thermal resistance is increased by the coating, so that only a low effectivity is obtained as compared to a graphite tube, if they are used as the heat exchanging element.
It is known from British Patent Application No. 2 069,342, U.S. Pat. No. 3,840,230 and other patents to cement layers of carbon fabric on frames of tennis rackets and other sport equipment and to thereby improve the strength and stiffness of a core which is surrounded by the reinforcing layers. U.S. Pat. No. 2,782,806, discloses a carbon tube which is impregnated with a synthetic resin and is connected to a flange, and to which a sleeve of fiber glass fabric is cemented in the region of the joint. The joint is impermeable to fluids and has great mechanical strength.